cleanup: Fix usages of non-constant format strings

[arjan-bal]

go 1.24 includes a vet rule that catches non-constant format strings: golang/go#60529. This causes go test to fail unless vet is disabled when using go1.24.

RELEASE NOTES: None

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.22%. Comparing base (f32168c) to head (89ac99e).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
stream.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7959      +/-   ##
==========================================
+ Coverage   82.04%   82.22%   +0.18%     
==========================================
  Files         379      379              
  Lines       38261    38261              
==========================================
+ Hits        31391    31462      +71     
+ Misses       5565     5506      -59     
+ Partials     1305     1293      -12     

Files with missing lines	Coverage Δ
internal/transport/handler_server.go	86.46% <100.00%> (ø)
xds/internal/balancer/clusterimpl/picker.go	97.00% <100.00%> (ø)
xds/internal/xdsclient/transport/ads/ads_stream.go	85.37% <100.00%> (+1.31%)	⬆️
stream.go	82.54% <0.00%> (+0.84%)	⬆️

... and 25 files with indirect coverage changes